A radio frequency identification (RFID) system has been known as a non-contact transmission system that wirelessly transmits information.
In general, the RFID system includes an RFID tag (also referred to as a “wireless tag”) and a reader/writer (RW). The reader/writer reads out information from an RFID tag and writes information into an RFID tag through wireless communications.
The RFID tags are classified into active and passive tags. An active tag includes a battery, and the active tag operates by power from the battery. A passive tag receives power from a reader/writer, and the passive tag operates by the received power.
Since the active tag includes the battery, the active tag is advantageous with respect to the communication range and the stability of the communication, compared to the passive tag. On the other hand, the active tag has disadvantages such that, since the configuration of the active tag is complicated, the downsizing is difficult and the cost tends to be higher.
Incidentally, due to the improvements in semiconductor technology in recent years, IC chips have further been downsized and the performance of the IC chips has further been improved. Accordingly, the communication range and the communication stability of the passive tags are being improved. Thus, it is expected that the passive tags will be applied in wider fields. For example, Patent Document 4 (Japanese Registered Patent No. 4723447) Patent Document 5 (Japanese Patent Laid-Open Application No. 2009-31893), and Patent Document 6 (Japanese Patent Laid-Open Application No. 2011-187075) disclose that passive tags are used for packages.
Further, it is expected that the RFID tags will be used for medicines such as tablets and capsules. The medicines such as the tablets and the capsules are sealed in a press through package (PTP), which is a type of heat seal packaging. In the PTP, one side of the package is covered with a resin sheet, and the other side the package is covered with an aluminum sheet.
For example, Patent Document 7 (Japanese Registered Patent No. 4787572) discloses a wireless IC tag that may be implemented in a package, for which a thin film of a metal such as aluminum is used as a sealing material.
Incidentally, various frequency bands are utilized for the RFID systems.
In an RFID system where a passive tag is utilized as an RFID tag, when a frequency band is a low frequency band and/or a high frequency band, a voltage is induced in the RFID tag due to an electromagnetic induction action between a transmission antenna coil of the reader/writer and an antenna coil of the RFID tag. An IC chip is activated by this voltage, and the communication is enabled. Namely, the communication is performed based on an electromagnetic induction method.
Therefore, in this case, the RFID tag operates only within the induction field caused by the reader/writer, and the communication range is several tens of cm.
On the other hand, when the frequency band is a UHF band and/or a microwave band, power is supplied to an IC chip of an RFID tag by a radio wave. Namely, the communication is performed based on a radio communication method.
Therefore, in this case, the communication range is significantly improved. The communication range is from 1 m to 8 m, approximately.
When the communication is based on the radio communication method, it is possible to read plural RFID tags simultaneously and to read a moving RFID tag. It has been difficult to perform simultaneous reading of plural RFID tags and reading of a moving RFID tag with the electromagnetic induction method. Therefore, it is expected that the RFID tags will be used in wider fields.
Incidentally, it has been known that, when a normal RFID tag is attached to a surface of a metal object or when water exists in the vicinity of the RFID tag, it is difficult to perform communication with the RFID tag.
Therefore, various types of RFID tags that can be used with a metal object have been proposed (cf. Patent Document 1 (Japanese Patent Laid-Open Application No. 2002-157565) and Patent Document 2 (Japanese Patent Laid-Open Application No. 2005-309811), for example). The proposed RFID tags can be attached to the surface of the metal object.
Additionally, Patent Document 3 (Japanese Patent Laid-Open Application No. 2008-284967) discloses a vehicle number plate. The vehicle number plate includes an RFID tag including a passive-type RFID chip and antenna patterns, wherein the passive-type RFID chip is disposed on an insulating film and does not require a power source, and the antenna patterns are disposed to sandwich the RFID chip; and a protection film formed on an entire surface of a conductive flat plate, wherein the entire surface of the conductive flat plate includes the conductive flat plate in which a slit has been formed, a filled portion formed of a non-conductive material that has been filled to the slit, and a top surface of the filled portion, wherein the RFID tag is disposed on a rear side of the conductive flat plate so as to cross the slit, and the insulating film is disposed to face the rear side of the conductive flat plate, wherein length L of the slit is expressed by L=λ/n, where the λ is a wavelength of a radio signal transmitted from the RFID chip and the n is an integer greater than or equal to 1, and wherein width of the slit is within a range from 7 mm to 9 mm when radio signals in a UHF band are transmitted and received, and the width of the slit is within a range from 2 mm to 3 mm when radio signals in a 2.45 GHz band are transmitted and received.